As shown in FIG. 11, a fuel assembly of a pressurized-water nuclear reactor is configured of fuel rods 100, an upper nozzle 101, a lower nozzle 102, guide tubes 103, an upper support lattice 104, an intermediate support lattice 105, a lower support lattice 106, and a foreign matter filter. The upper support lattice 104 and the intermediate support lattice 105 are mechanically joined to the guide tubes 103, and the lower support lattice 106 is mechanically joined to lower ends of the guide tubes 103 together with the lower nozzle 102. The upper support lattice 104, the intermediate support lattice 105, and the lower support lattice 106 hold the fuel rods 100. As shown in FIG. 12, the upper nozzle 101 is joined, by welding, to upper end portions 108 of sleeves 107 attached to the upper support lattice 104. Moreover, the sleeves 107 and the guide tubes 103 are mechanically joined to each other, respectively.
In such a fuel assembly, there is a case where the upper nozzle 101 of the fuel assembly is required to be detached in a power plant in order to, for example, replace the fuel rod 100 which has been damaged during an operation. Accordingly, the fuel assembly must be designed so that the upper nozzle 101 is detachable. However, the upper nozzle 101 is conventionally joined to the sleeves 107 by welding as described above, and therefore, cannot be easily detached and attached in the power plant.
In this respect, a coupling structure of a fuel assembly using a nut 111 is known as an upper nozzle attachment/detachment structure enabling attachment and detachment of the upper nozzle 101. In this structure, as shown in FIG. 13, a male screw thread 109 is formed on the upper end portion of the guide tube 103. The guide member 103 is fastened and fixed in a clamping manner to the nut 111 having a female screw thread 112 formed on its inner surface through a locking cup 110 provided in the upper nozzle 101 (see Patent Document 1 mentioned below).
Further, a coupling structure of a fuel assembly using a locking ring is known as the upper nozzle attachment/detachment structure. In this structure, a projecting portion is provided on the guide tube 103. The locking ring (omitted from the drawings) is attached to the upper nozzle 101 and is rotated with the projecting portion being inserted into an opening portion of the locking ring. Thereby, the projecting portion of the guide tube 103 overlaps a receiver portion of the locking ring, and the guide tube 103 and the upper nozzle 101 are thus coupled to each other (see Patent Document 2 mentioned below).    Patent Document 1: Japanese Patent Application Publication No. Sho. 59-12383    Patent Document 2: Specification of U.S. Pat. No. 5,844,958